1. Field of the Invention
This invention relates to an electrical circuit such as a Wheatstone bridge with at least one resistance-adjusting portion.
2. Description of Related Art
Such circuits are widely used to measure the deformation of mechanical parts through the variation in the resistances of the resistors which they include and which are deformed at the same time as the part on which they are bonded or deposited. An important advantage of a Wheatstone bridge is its high stability over long periods and with respect to changes in temperature. It is however necessary to adjust the resistances to balance the bridge, i.e. to obtain a null signal between the measuring terminals when no load is applied. This is why bridges are provided with additional resistors which can be adjusted to balance the bridge and possibly its thermal sensitivity. Several methods have already been developed to effect these adjustments.
In U.S. Pat. No. 4,945,762, current-limiting resistors and adjusting resistors are placed in series. During normal operation of the circuit current flows simultaneously through the adjusting resistors and the current-limiting resistors. When an adjustment is to be made, current is caused to flow in a reverse direction in a pulsed mode so as to bypass the bridge resistors and current-limiting resistors and flow through branches formed by diodes which are conducting in this one direction only. The current passes through the adjusting resistors only, which resistors are then subject to such an intensity of current that migration of constituent atoms occurs towards the surrounding substrate, which modifies their resistance.
In other proposals, adjusting resistors connected in parallel are added or removed. A disadvantage is that the carrying out of such operations entails physical/chemical non-uniformities in the conductive material which forms the bridge or circuit, for example by oxidation. The different materials or alloys usually have characteristics which change differently with time and which also behave differently in response to variations in temperature, as the thermal behavior is also a characteristic of the material. Measurement quality is compromised by these various sources of inaccuracy.